The present invention relates to a control circuit for a solid fuel furnace which includes a combustion chamber, a device for delivering solid fuel and a device for delivering combustion air to the combustion chamber. The heat from combustion can be used, for example, to heat warm air or water which, in turn, are conveyed to a comfort zone. The furnace includes thermo-sensitive means which establishes the set point temperature for the furnace and controls the delivery of fuel and combustion air to the combustion chamber during periods when the set point temperature is not met to maximize combustion efficiency and minimize fuel consumption and smoke and soot formation. The invention also includes a thermo-sensitive means responsive to flue gas or furnace temperature to control delivery of fuel and combustion air to the combustion chamber during periods when the set point temperature is met to maintain the fire bed ready to supply heat upon demand.
In the typical solid fuel furnace, the fuel delivery means and the means for delivering combustion air to the combustion chamber operate together. When the set point temperature is met, for example, when the temperature of the comfort zone (room) heated by the furnace reaches the set temperature of a thermostat in that zone (room), both the fuel delivery and combustion air delivery means turn off. This often leaves a large quantity of raw fuel in the fuel bed so that large quantities of volatiles in the raw fuel are released into the furnace atmosphere. Since the combustion air delivery means has been turned off, there is a deficiency of combustion air in the furnace atmosphere causing heavy smoking and sooting which contaminates the air, furnace surfaces and parts, and the raw fuel in the fuel bed. This results in air pollution, reduction in furnace efficiency, and excessive fuel consumption.
In furnaces of this type, it is desirable also to provide a "holdfire" system to maintain the fire bed ready to supply heat upon demand, e.g., when the temperature in the comfort zone falls below the set point. The "holdfire" system should function only when necessary to avoid excessive fuel consumption.